Could you create a large Helium balloon with a high Isp ion drive, and power the engine with a ground based microwave power link? The balloon would be spherical with the top portion made from mylar to create a giant parabolic antenna. The rectenna would be suspended inside the balloon at the focal point of the shaped mylar.

Float the system to 30+km as a balloon then power the engine from a large ground based microwave transmitting station. Use that power to fire the engine to lift the balloon the rest of the way to GEO or space.

i'm skeptical that you could get enough thrust from an ion drive or other type of electric propulsion to actually raise the balloon, since the buoyancy would have to increase with increasing altitude in order for you to not just fall back to earth. however, i like the idea of using the balloon as an antenna, for example for beaming power to a DSS.

That was my original thought, but then I ran across a discussion of a "lifter" which I believe works on the same principal as an ion engine.http://en.wikipedia.org/wiki/IonocraftIt can lift it's own weight at sea level at 1W/gm, to it's not inconceivable at first glance. The problem comes when the pressure drops the lifter will have less to work with. At 30K you only have something like 1% of the atmosphere, so you may only get 1% of the lift, but since theoretically we could have unlimited power, that may just drop the efficiency not the overall thrust. I wonder if you could do some kind of ion scoop to gather the thinner atmosphere at higher altitudes.Most of the ion engines I've heard of are designed for efficiency of power and highest Isp, I wonder if the extra power and larger "fuel" reserves would allow larger thrust engines to be developed that could make this work.

Ion drives rarely work at that sort of pressure, and if they do they're inefficient.

The trouble with any beamed power system is that you have to accelerate up to speed before passing out of the horizon. Launching and beaming power from a Dark Sky Station helps, as it increases the distance to the Horizon (due to the higher altitude), but t still won't be possible to get a manned craft to orbit (acceleration problems). That doesn't mean it can't be used as a high efficiency first stage.

//Ion drives rarely work at that sort of pressure, and if they do they're inefficient.//Couldn't you use "lifter" tech to ionically lift the balloon until the pressure drops low enough to use the ion drive?http://en.wikipedia.org/wiki/Ionocraft

QED I believe your on the right track. I cannot yet comment but I do look forward to doing so in the near future. I can say some basic issues are being overlooked. That have solutions and that microwave power is not needed but is something we are looking into doing some R&D on. There is a way to get a high ISP Ion thruster in the upper atmosphere and I'll say that EHD thrusters are useful up to the point were the Ion drives will work.

i'm really not seeing it. your "unlimited power" assumption is a) faulty, since there will be some practical limit of what you can actually send and b) logistically infeasible - in order to use any type of electric propulsion to generate large amounts of thrust you would need massive (order of megawatts or greater) amounts of power and that will be expensive and probably not available to you. what you're talking about with lower ISP higher thrust is simply not available. the only type of electric propulsion capable of achieving the required impulse is vasimr (well, possibly arcjets or similar that operate on a heating mechanism rather than ions or plasma), and vasimr is no where near ready for something like that. ion drives do NOT scale upwards in power - they have a small range of powers they can operate under. SPTs and hall effect thrusters have the same limitation. if you want infinitely scalable power you have to go to a type of electric propulsion that's either a) vasimr or b) theoretical and not built yet. developing a functional, experimental, electric propulsion unit is an epic task better suited to high energy physics labs or people with nasa contracts than n-prize teams. i won't make any statement about arcjets or similar EPs that operate by heating a gas, these may be worthwhile and capable of lifting a balloon from high altitude into orbit, but no ion or plasma based propulsion is within 10 years of being able to do so, and probably 20 or more. also, arcjets have fairly low ISPs (under 1000s) so you're not gonna save much mass by using one, and you will waste a lot of development money.

Obviously "unlimited power" is incorrect, what I mean is that the power is not constrained by that which is available available on board, so it is relatively unlimited in that is is only limited by the amount of power that can be received by the ship. The basic idea here is a combination of several well accepted ideas: helium balloons, microwave power beaming, "lifter" technology and ion engines. Balloons can get you to 30km. Lifter technology has been shown to work, though it takes a lot of power and doesn't work in complete vacuum. Ion engines work, but only in partial vacuum and produce low thrust and need a decent amount of power to run.If you start with a system which relatively has very low mass and relatively high power and lift it to 30km with a balloon such as a rectenna, an ion engine. Can you then still employ the lifter tech to push you out till the pressure drops low enough so that the ion drive will work effectively? There is still some atmosphere at low orbit so both the balloon and the the lifter tech should still work even if at a massively reduced efficiency. The real question is whether an ion engine can produce the thrust of even a fraction of its own mass so that it can provide any real contribution once the atmosphere gets thin enough.An alternate plan is to take the common route to orbit and use the lifter tech and then the ion drive to head east to gain speed to get to orbit, but that would require multiple ground power stations. Or to replace the ion engine with one of the other technologies you mentioned.

I agree with the idea proposed here about using EHD/MHD/LHD (Electric/Magnetic/Lorentz) propulsion to get upto the top of the atmosphere, and then use a different propulsion from there to get to orbit, such as VASMIR or PIT.

The thing is that if you can create an ionized plasma envelope around the craft, then its drag will be much lower. At the same time, you can use that same ionized flowstream for the EHD/MHD/LHD propulsion. And while you're doing that, you could collect and compress gas for you to use later for direct ion propulsion in the vacuum, under PIT. 3 birds with one stone.

As for powering it all, I'm thinking an onboard nuclear reactor would be best, like a SAFE400, or more likely a particle-bed reactor..

Designing a spacecraft is going to involve compromises. I feel the best compromise is having a vehicle that is extravagant on energy consumption while being frugal on mass expenditure. You can't just pull more mass out of nowhere, to further your propellant supply in the vastness of space. But you can grab more energy out in space, since there's a lot of raw energy coursing through it, even close to the Earth. Plus it's easier to keep lots of energy onboard compactly as nuclear fuel.It's really a question of using nuclear energy for launch in a way that doesnt' contaminate the Earth, and to me that automatically means nuclear-to-electric conversion, at a high rate of power.

The high rate of power probably rules out Stirling Engines with their high efficiency.I'm imagining AMTEC could be a good candidate (Alkali Metal Thermal to Electric Conversion)It's lightweight, thermally regenerative and can take temperatures of upto 1500degC for conversion into electric power, and is being studied for space propulsion systems.I'm wondering whether it could be useful for a launch vehicle.

Specifically for a launch vehicle, I'd want a particle-bed reactor, operating in burst mode.Burst mode power would certainly be muscular enough for launching to orbit.

The concerns with particle bed reactors again are typically around thermal coupling with the flowstream and contaminating it, but again nuclear-electric would avoid that problem.

I've also wondered about tokamaks and 'burning plasma'As we know, breakeven fusion power has not been achieved yet - but burning plasmas certainly have.If you could have a burning plasma that lasted for maybe 20 minutes, I think that would be enough to drain energy off of, and divert for propulsion purposes to achieve orbit.

Lastly, I'd also like to mention aneutronic fusion, because at least it requires the least shielding, since it produces no neutrons.If aneutronic fusion could be done with Boron, then it would be a great leap forward not just for energy supply here on Earth, but for achieving space travel as well.